The photosensing area of a frame transfer CCD imager includes a plurality of parallel channels in and extending along the surface of a substrate of semi-conductor material. The channels are generally of N type conductivity in the substrate of P type conductivity. Between the channels are channel stop regions which are of N- type conductivity and which extend along the surface of the substrate. Within the channel stop regions are blooming drain regions of N+ type conductivity. L. F. Wallace in U.S. Pat. No. 4,362,575, issued Dec. 7, 1982, entitled "Method of Making Buried Channel Charge Coupled Device With Means For Controlling Excess Charge", describes a method of making such a CCD imager with the blooming drains being self-aligned with the channel stop regions. The Wallace method includes forming a silicon oxide masking layer on the surface of the substrate and coating the masking layer with a layer of a photoresist material. Using standard photolithographic techniques, openings are formed in the resist layer where the blooming drains are to be formed, and the areas of the masking layer exposed by the openings in the resist layer are removed with a suitable etchant to form openings to the surface of the substrate. N type conductivity modifiers, such as arsenic or phosphorus, are then implanted into the exposed areas of the substrate to form the blooming drains. The openings in the masking layer are then made larger by etching away the masking layer under the resist layer to expose areas of the substrate surface on each side of the blooming drains. The resist layer is then removed and P type conductivity modifiers, such as boron, are implanted into the exposed surface areas of the substrate to form a highly doped P type region around each blooming drain. The masking layer is then removed and N type conductivity modifiers are embedded into the entire area of the substrate surface. This forms the N type channels between the blooming drains and compensates the P type region on each side of the blooming drains to form N- type channel stop regions between each channel and each blooming drain.
E. D. Savoye et al. in patent application Ser. No. 455,332, entitled "Imaging Array Having Higher Sensitivity and a Method of Making Same" filed Jan. 3, 1983 and assigned to the same assignee as the present application, discloses a frame transfer CCD imager including a highly conductive P type potential barrier region beneath each blooming drain region to direct charge carriers generated in the substrate towards the channels. Savoye et al. make the CCD by the same method as described by Wallace with one additional step, i.e., the blooming drains are formed but before enlarging the openings in the masking layer, P type conductivity modifiers, such as boron, are implanted into the substrate through the blooming drains to form the potential barrier regions under the blooming drains.
The method disclosed by Wallace and Savoye et al. are suitable for forming the CCD imagers having self-aligned blooming drains in the channel stop regions. However, it would be desirable to reduce the number of steps necessary to form such devices. Also, the etch back of the masking layer to define the channel stop regions forms relatively wide channel stop regions. It has been found that the relatively wide channel stop regions are not only essential for the operation of the device, but they also take up additional space which enlarges the entire CCD imager.